As we saw today, Rav Ashi differs with Ula's answer in that the coin provided by the shulchani is a real matbeia, yet there is no problem of chalipin with tiv'a due to the fact that he has money at home (based on tosfos understanding of the sugya). Acc to Rav Ashi, though, why does the money returned by the purchaser (the guy paying his workers) have to be non minted coins (it seems acc to Rashi that this is not the case)? As we said in shiur, since you are "purchasing" maos for the dinar, why cant we understand that he is paying back with real money as well?
Based on maharsha in an earlier tosfos, perhaps the answer is that this allows us to learn the sugya even acc to man d'amar that אין מטבע נקנה בחליפין
Perhaps, Tosfos is telling us that money for money, even if the money is different amounts would still involve se'a be'sa and would not be considered a sale of money? The language used in tosfos , though, calls this a matbea and not an unminted coin, which seems to not be what Rav Ashi answers.